1. Field of the Invention
The present invention relates to an epoxy resin composition having high neutron shielding ability and excellent transparency, and also related to transparent shielding materials for neutron obtained by molding and curing said resin composition.
2. Description of the Prior Art
Recently, along with a wide range of nuclear industry and its application, for instance, nuclear power stations or nuclear fusion facilities such as nuclear reactor or fast breeder reactor, or neutron treatment institute used for medical treatment, the requirement for development of materials which are possible to shield neutron harmful to human body and protect from the adverse affect of neutron generated from said facilities is becoming more important. The development of a transparent shielding material which captures neutron effectively has been desired for a long time for the purpose to be used in the following applications; the remote controlling unit for above-mentioned facilities, various kinds of safety control tools for checking goods for operators inside the above mentioned facilities, protecting tools to prevent operators from neutron exposure in the emergency case such as neutron emission, or emergency rescue facilities and ambulance cars for emergency trouble such as neutron leakage.
There are two different kinds of radiation that are emitted in the above mentioned facilities and stations, that is, xcex3 ray and neutron. Further, the secondary radiation from the above mentioned radiation must be considered from the standpoint of the shielding. Therefore, shielding materials also have to have shielding characteristics against this secondary radiation. Since the damping characteristics of xcex3 ray and neutron indicate different values according to each substance, various combinations of many substances have been investigated. Concretes are generally in use as effective shielding material for both of xcex3 ray and neutron. On the other hand, as the shielding materials for xcex3 ray, lead can be used. However, as the mechanical strength of the medal is not so strong, the usage of lead is limited. Furthermore, water can be widely utilized as shielding material for thermal neutron and xcex3 ray because the hydrogen atom content in water is high (refer to pages of 220 to 221 of Atomic Power Handbook, edited by Tadakazu Asada, Ohm Co., Ltd., 1976). Because water can be flowed by pump through the flowing section inside a nuclear reactor and is used as heat transfer medium and shielding material for neutron. Referring to the concrete, it can be said to be an excellent material for shielding neutron and is cheaper than other materials for shielding and the mechanical strength required to form a structure can be easily obtained. Therefore, it can be utilized for various parts of the facilities. However, structure of concrete itself is not transparent and it is impossible to watch the inside of the facility shielded by the concrete. In this indicates that concrete is different from watching through glass. As to shielding materials of neutron, materials of which the compositions are polyethylene resin and also relatively large content of boron compounds are known (Japanese Patent Publication 62-49305). Further, molding compounds which are composed mainly of boron minerals using thermoplastic hydrocarbon resins containing high number of hydrogen as binder (Japanese Patent Laid Open Publication 52-106097) are known. In case where epoxy resins are used as main component, the shielding material in which a fast neutron moderating layer composed mainly of epoxy resin with fine iron powders, a thermal neutron capturing layer containing boron (Japanese Patent Publication 4-47800), the cured products made by curing mixture containing epoxy resin, polyethylene, and inorganic boron compound with some kind of amine type hardeners (Japanese Patent Publication 4-67160) and cured products product of epoxy resin and filler with hardener in order to be used for nuclear fusion reactor""s a supporting structure (Japanese Patent Laid Open Publication 60-387) have been proposed. However, regarding to transparency of neutron shielding material which this invention claims, any useful proposal has not yet being made now.
Aim of the Invention
The most important factors required for neutron shielding material are lower price and easier handling it. The shielding mechanism to capture neutron or thermal neutron is evaluated by the reduction of the neutron energy. It said that materials consisting of lower atomic number elements are more effective as neutron shielding material. Therefore, hydrocarbon compounds containing relatively large number of hydrogen atoms (such as, paraffin, polyethylene resin, epoxy resin or acrylic resin) are in use as shielding material for the structural parts of radiation treating facilities. As neutron shielding materials, only opaque structural parts which are made by blending an inorganic filler with some of chemical compounds, which have capability of radiation shielding and capturing performance of neutron such as boron added hydrocarbon compound are usually available. Regarding to any transparency in shielding material of neutron, any useful proposal has not yet been made at the present time. In the meantime, as a concrete example of transparent shielding material, lead glass with heavy density can be mentioned. However, its neutron shielding performance is low, and results in requiring very thick layer of it to attain sufficient shielding effect. Therefore, when it will be used, its actual weight will become very heavy. Furthermore, the durability of lead glass to water is very low and also the resistance against external shock is poor. Considering the above-mentioned present situation, the development of new shielding materials for neutron having the following features has been expected for a long time. Namely, it is figured that a material has excellent transparency like glass, good neutron shielding effect, strong mechanical strength, and good heat resistance not softened during a long term irradiation with lighting (heat wave), further, reduced in weight with reasonable price.
In view of the above mentioned situation, the inventors of this invention have conduced intensive studies to develop a neutron shielding material which is excellent at transparency and is easily molded, and it enabled us to have developed a transparent neutron shielding material which has high neutron shielding and its capturing capability. The aim of this invention is to provide an epoxy resin composition with neutron shielding capability for making a large size of shielding material with over 50 mm thickness economically, which is the required enough thickness to give sufficient neutron shielding performance, and another aspect of this invention is to provide a large size transparent molded product for neutron shielding.
To attain the above-mentioned aim, the shielding material has to have following physical properties. Firstly, the material must be transparent. Secondly, the cured epoxy product must be excellent at shielding capability for delayed neutron. That is, it has the performance to moderate level of neutron energy. Thirdly, the shielding material has to be good for the mechanical strength and heat resistance. Fourthly, the shielding material is required to be thick and be molded in the form of bigger sizes.
In the present invention, the following methods and procedures were selected to meet the above-mentioned requirements.
Regarding the transparency mentioned as the first important requirement, it was able be solved not to blend any opaque inorganic compound for shielding for neutron, but to select a transparent epoxy resin and a hardener as starting materials and then to make its epoxy resin composition of which the curing can be made at ambient temperatures and subsequently any shape distortion will not occur.
As method to improve the neutron shielding capability, which is the second important physical property, it was adopted both to select epoxy resin and hardener, which contain a lot of hydrogen atom, which is the lowest atomic number. Subsequently, the neutron shielding performance was improved. When the number of hydrogen atoms per one molecule is relatively high, the fact that neutron shielding capability can be improved was confirmed by actual experiments, and the experimental results showed that the shielding material was decreased in its thickness thinner than conventional ones in order to attain the same shielding effectiveness. Consequently, it was possible that the neutron shielding material was lighter than usual. Further more it can handle easily.
Regarding the improvement in mechanical strength and heat resistance of which the above cured epoxy product has which is the third requirement, the cured product having desirable mechanical strength and heat resistance, which can be used practically was developed by use of the epoxy resin as thermosetting resin.
The shape of structures to which the shielding material of this invention is applied depends on their various applications and the molded products are desirable to be thicker in order to get a designed shielding capability. Therefore, with regarding to the fourth requirement, the method to cure the epoxy resin composition has required usually to control heat of curing reaction. In the epoxy resin composition of this invention, it is indispensable to control and remove properly heat of curing reaction in the molding step of the epoxy composition because any inorganic compound with high heat capacity to absorb the heat is not blended with the composition. If the above heat can not be controlled, a kind of strain molding phenomenon will occur and result in making not uniform molded body and subsequently bad transparent one, and in case where a transparent organic mold is used for molding, the mold itself is distorted in shape by the heat of curing reaction and consequently the molded product becomes distorted in shape. And there are cases where are found fish eyes or a kind of foams in the molded product to incur decrease in its shielding performance and its subsequent impractical use. Concerning the above facts, the inventors had studied to solve the problems and completed the following process method for molding.
(1) Epoxy resin composition is defoamed previously by vacuuming before the molding process,
(2) the defoamed composition is divided into small portions and casted into a mold intermittently,
(3) preventing foams from getting caught in the melt composition at melt flow in gate of the mold, the composition is hardened at ambient temperatures by use of an external removing method for heat of curing.
When a metallic mold is used, it is necessary to part the molded product from the mold. However, when a transparent organic mold is used, the transparent shielding material can be molded as one body with the mold to have been adhered tightly each other, and a multi layered shielding material can be obtained. Namely, said new multi layered shielding material characterizes that any parting step is not required.
That is, the essentials of this invention are use of a transparent epoxy resin composition with excellent neutron shielding capability, which can be cured at ambient temperatures. The said epoxy resin composition consists of an epoxy resin not to require any opaque inorganic compound for heat absorption of curing, and a hardener and the viscosity of said composition is lower than 7000 mPaxc2x7s at ambient temperatures.